1. Field of the Invention
The present invention relates generally to photoconductive switches and more particularly to optically initiated silicon carbide (SiC) and other high voltage switches.
2. Description of Related Art
Particle accelerators, for example dielectric wall accelerators (DWAs), critically depend on high voltage, high current, fast closing switches that can be activated with nanosecond precision. Photoconductive switches offer the most potential in terms of handling such high voltages and high currents with minimum inductance, rapid closure, precise temporal control and the possibility of long life. Photoconductive switching is a technique in which optical energy is applied to a semiconductor material, and the optical energy creates carriers within the semiconductor, rendering it conductive.
The materials that have been used to date for photoconductive switch applications include Silicon and Gallium Arsenide (GaAs). The difficulty with these materials is that various failure mechanisms occur, even at modest parameters. Further, the optical absorption depth for these materials is low, so the carriers are forced to flow in a very thin area of the material bulk just below the surface. The principal issue with photoconductive switching has been short lifetime resulting from overstressing current and voltage conditions.
Additionally, silicon carbide (SiC) has long been a promising alternative candidate for use as a photoconductive switch material. Only very recently, however, has this particular material been commercially available in sizes and purity that merit application as a high voltage switch. SiC material has high dielectric breakdown strength, greater than that of most solid materials (about 4 MV/cm); high thermal conductivity (comparable to that of copper); and low optical absorption. Thus, with the availability of single crystalline Silicon Carbide, a new class of switches is possible.
While promising, even SiC is subject to failure due to high electric fields locally produced where the electrodes separate from contact with the photoconductive substrate. A prior art photoconductive switch, having a SiC photoconductive substrate and two opposing electrodes, typically has a meniscus formed at the metal contact between the electrode and substrate surfaces. The meniscus refers to a small blob of indium solder that is used to bond an electrode to the substrate surface. The magnitude of the electric field on the contact surfaces has a spike in magnitude at the triple points. The triple point is a region where the electrode edge, the SiC wafer, and another material like an insulating oil all come together. The indium solder has some vertical height to it that permits oil to come in contact with both the electrode and wafer, causing the triple point. Various methods have been employed to reduce and minimize these fields at such “triple points,” including filling the space between the electrode and substrate with a high permittivity material. However, there is still an electric field spike, albeit with less magnitude, at the triple point of electrode-substrate separation.
Copending U.S. patent application Ser. No. 11/586,468 describes a photoconductive switch with a photoconductive substrate having opposing electrode-contacting surfaces and a facet optically connectable to an optical source for receiving optical energy; two electrodes electrically connected to the electrode-contacting surfaces of the substrate, for applying a potential across the substrate; and two field-grading liners formed on the substrate surrounding the electrode-contacting surfaces, for grading the electric fields therealong. The field-grading liners may be adjacent to the electrode perimeters, or they may be adjacent to the substrate perimeter, but they are integrally formed into the substrate. The field-grading liners may be made of high permittivity materials or conductive or semi-conductive materials; a suitable material is silicon nitride. The liners may be formed as doped sub-surface layers of the substrate, extending into the substrate about 1 micron deep. Optionally, the substrate can be a multilayer having at least two photoconductive layers separated by a divider layer, with the divider layer composed of conductive and semi-conductive materials. While the field-grading liners may reduce the electric field effects, they add a level of complexity since they must be fabricated into the photoconductive substrate. Another problem with resistive liners is that they usually must be custom configured for a particular pulse width of applied voltage and will not work for arbitrary waveforms.
The photoconductive substrate shown in Ser. No. 11/586,468 may have two opposing concavities and the two electrodes may have convex surfaces contactedly seated in the two concavities. These electrode shapes could be used in the present invention.
What is needed therefore is a photoconductive switch for high voltage applications such as for particle accelerators, preferably implemented with SiC or other photoconductive materials such as GaN, that minimizes or at least reduces the high magnitude electric fields at the points of electrode-substrate separation. A switch design that does not require alteration of the photoconductive substrate would be highly advantageous.